The present invention relates to a method and apparatus for chemical vapor deposition (CVD) of thin films on a semiconductor substrate which uses an ultrasonic nozzle based liquid delivery system for liquid precursor solution injection.
MOCVD (metalorganic chemical vapor deposition) techniques have come into increasing use in recent years for depositing thin films, due to the growing versatility and availability of precursors and improved environmental advantages over the halogenated compounds used for conventional CVD. Most commonly, the films are deposited by dissolving the metalorganic reactant, typically a metal alkoxide, in a carrier gas, which is carried through heated lines to a heated substrate. The metal alkoxides decompose by pyrolysis, leaving the metal oxide. While this method has been used to produce excellent epitaxial films, it has the disadvantages of complexity: the precursor must be heated to fix its vapor pressure, the gas flow rate must be carefully controlled and the carrier lines must be heated to prevent condensation before reaching the reactor. Additionally, this conventional method cannot be used with certain low-volatility precursors that decompose upon heating.
Various techniques have been used to overcome some of these difficulties. Titanium dioxide films have been grown by a direct liquid injection method in which the precursor, in dilute solution, is introduced into a heated antechamber, where it evaporates and diffuses to the heated substrate below. However, this process is difficult to control, both in terms of injection and evaporation rates.
Other examples of liquid or solution precursor CVD are based on spray pyrolysis techniques and are not limited to metalorganic precursors. In some cases, a nebulizer is substituted for the gas bubbler and the carrier gas brings a fine mist to the reaction chamber where it evaporates. In other cases, a spray solution is injected directly into the reaction chamber, generally at atmospheric pressure.
Another type of liquid source CVD method is disclosed in an article by L. D. McMillan et al. entitled "Liquid Source CVD", Integrated Ferroelectrics, 1992, Vol. 2, pp. 351-359. This method employs an ultrasonic cavity to mix a liquid source with a gas stream and create a fine mist for direct injection into a vacuum chamber. As discussed in the article, this method could not, however, achieve uniform film depositions onto substrates after the liquid source mist has been introduced into the vacuum chamber unless a barrier plate was placed in close proximity to the substrate surface, the substrate was rotated and the mist was injected through a specially designed circular manifold such that the mist became confined to a small region of vacuum directly over the substrate.
In view of the foregoing, a need exists for an improved liquid source CVD technique that can achieve uniform film depositions on all sizes of substrates without the need for expensive or complex structural arrangements.